Heretofore, it is known to convert various hydrocarbon materials by contacting them with a solid acid catalyst, particularly an acid type (proton type) zeolite, whose alkali is removed by acid or an ammonium salt, and initiating cracking, isomerization, disproportionation, aromatization or the like reaction.
As a representative example, conversion of gas oil, heavy oil or the like into a gasoline fraction is widely conducted in petroleum refining. Further, there is proposed a method for converting light hydrocarbons into aromatic compounds by using a proton type ZSM-5 zeolite in Japanese Patent Unexamined Publication No. 41322/1974 (corresponding to British Patent No. 1381427), Japanese Patent Unexamined Publication No. 49233/1975 (corresponding to British Patent No. 1394979), Japanese Patent Unexamined Publication No. 4029/1975 (corresponding to British Patent No. 1442850) and the like. There is proposed a method for converting light hydrocarbons into lower olefins and aromatic hydrocarbons by using a proton type ZSM-5 zeolite in Japanese Patent Unexamined Publication No. 222428/1985 and Japanese Patent Unexamined Publication No. 130236/1991 and the like. There is proposed a method for converting light hydrocarbons into lower olefins by using a acid type zeolite containing silver in U.S. Pat. No. 4,361,502 and Japanese Patent Unexamined Publication No. 184638/1990.
There is proposed a method for producing aromatic hydrocarbons from light hydrocarbons in a catalyst bed having specific temperature distributions by using a specific zeolite type catalyst in International Patent Application No. PCT/JP95/01059.
As a method for obtaining lower olefins and monocyclic aromatic hydrocarbons from hydrocarbon materials, heretofore, a pyrolysis method is widely used. However, the method needs severe reaction conditions because of use of pyrolysis so that a lot of methane, which is difficult to use as a raw material for petrochemistry, is produced as a by-product. Further, the method has problems that since the ratio of product yield of olefins such as ethylene, propylene and the like and monocyclic aromatic hydrocarbon such as benzene, toluene and the like is substantially fixed, product flexibility is poor because of its yield structure, and the total yield of olefins and cyclic aromatic hydrocarbons (effective product yield) does not exceed about 60% by weight.
Therefore, in order to solve these problems of the pyrolysis method, heretofore, a method for catalytically cracking hydrocarbons by using a solid acid catalyst, particularly an acid type (proton type) zeolite catalyst has been studied. However, none of the known methods is satisfactory as a method for efficiently and stably obtaining both lower olefins (olefins having 2 to 4 carbon atoms, that is, ethylene, propylene and butene) containing ethylene, which is useful as a raw material for petrochemistry, as a main component and cyclic aromatic hydrocarbons (aromatic hydrocarbon having 6 to 9 carbon atoms, which contain benzene and alkylbenzene), and obtaining a yield of lower olefins containing ethylene as a main component higher than that of cyclic aromatic hydrocarbons.
For example, a catalytic conversion method using Y type zeolite produces a lot of paraffins, which are low value, and in addition produces extremely little aromatic hydrocarbons. A method using a proton type ZSM-5 zeolite generally provides a relatively high yield of hydrocarbons, a cracking gas composition having mainly light paraffins such as ethane, propane and the like, and poor selectivity of lower olefins. In order to improve selectivity of lower olefins, a method for introducing copper or silver is known (Japanese Patent Unexamined Publication Nos. 1413/1990 and 184638/1990). However, the method improves the yield of propylene, but provides low yields of ethylene and aromatic hydrocarbons.
Further, in the above method for catalytically cracking hydrocarbons by using the acid type zeolite, since coke is accumulated on the catalyst, a regeneration operation is often necessary to remove the coke by burning it off. The above method has a problem that the catalyst is permanently deactivated because of the repeated regeneration operation. This phenomenon occurs because the zeolite is hydrolyzed by steam, which is generated by burning of coke, and aluminum is released from the zeolite crystal so that proton active sites disappear. When the proton type zeolite is used in this kind of a reaction, this phenomenon is an unavoidably big problem. In order to solve this problem, there is proposed a method for inhibiting the above decrease of activity by introducing silver into the proton type zeolite (Japanese Patent Unexamined Publication No. 117584/1984 and U.S. Pat. No. 4,845,063). However, this method has problems such that the effect of the inhibition is not sufficient and besides, with respect to yield, the selectivity of lower olefins is low. Further, in order to inhibit accumulation of coke, there is proposed a method for contacting a zeolite with steam under specific temperature distributions in International Patent Unexamined Publication No. WO95/09050.
The present invention provides a stable catalyst for conversion of hydrocarbons, which is capable of obtaining olefins containing ethylene as a main component and monocyclic aromatic hydrocarbons with good balance and a high yield, and which is scarcely deteriorated by steam having a high temperature. Further, the present invention provides a production method by using the above method.